The rechargeability of battery systems, amongst other things, is affected by the presence of ionic species that interfere with the reversibility of reduction and oxidation processes within the electrochemical cell. For example, the rechargeability of manganese dioxide cathodes in battery cells with zinc as anode and KOH as electrolyte is affected by dissolved zincate species, where the zincate interferes with the reduction and oxidation processes involving manganese dioxide thereby reducing the cycle life of the battery. Zincate, formed at the anode, migrates through the separator to the cathode side of the battery where its known deleterious effect occurs. There has, therefore been a need for a separator or electrode coating material that selectively inhibits the passage of zincate ions to the cathode.
Past membrane separators have used microporous membranes which are not species selective thereby enabling the crossover of electroactive ions to take place which leads to self-discharge, reduced cycle life and loss of coulombic efficiency of the cathode.
There has, therefore, been a need for membrane material to be used as a separator or electrode coating material with properties that enable it to overcome the problems of a separator that is not species-selective. In addressing these problems, a separator or coating material should be ion-selective, that is, enables the free passage of certain ions between electrodes but inhibits the passage of others. In the manganese dioxide battery system, there is a need for a separator or coating material that enables the movement of hydroxide but inhibits the passage of zincate and other complex ionic species. Zincate is a relatively large negative ion compared to a negative ion such as hydroxide. The mechanism of exclusion may be by the Donnan exclusion method (surface charge) and/or by physical closure of a fraction of membrane pores. The membrane must, however, maintain a low resistivity to maintain battery efficiency and be ion-conductive at the pH of the system.
As well, there is a need for a membrane material that is inexpensive to make and is stable in the concentrated electrolyte of the battery system. In particular, there is a need for a polymer material soluble in common organic solvents, such as methanol, but which is not soluble in most battery electrolytes, such as potassium hydroxide. The polymer membrane must also be able to effectively bind an ion-selective coating.
Past battery separators have used materials such Nafion.TM. (a sulphonated polytetrafluoroethylene) or fluorosulphonated teflon.TM.. In addition to being expensive, these materials are unsuitable as ion selective separators for commercial battery systems. In particular, Nafion is primarily a protonic conductor, that is, more conductive in an acid environment. Furthermore, it has a low ion exchange capacity and is not readily bonded to a polymer backbone which makes it unsuitably fragile for use in a commercial battery system.
Other separator materials such as cellulose are unstable in strong alkaline electrolytes, such as 9 M KOH.
Earlier work by Dzieciuch et al (J. Electrochem Soc. 135 2415-2418 (1988)) showed that a serious loss of rechargeability resulted due to zincate ions diffusing to the manganese dioxide based cathode. Test results with Nafion, Daramic.TM. and Celgard.TM. separators illustrated that separators which impeded the transport of zincate enabled a large number of cycles to be attained with a significant amount of the two electron capacity remaining.
Arnold and Assink (Journal of Membrane Science, 38 (1988) 71-83) developed sulfonated polysulfones membranes for use in a flow battery. These membranes demonstrated appropriate stability, resistivity and selectivity for a flow battery.